1. Field of the Invention
The present invention relates to an airbag apparatus for an automobile for protecting a person in an automobile such as a car; for example, a driver or a passenger sitting in the front passenger seat, from impact upon head-on or side collision of the automobile to thereby ensure safety of the person. More particularly, the invention relates to an improvement of fracture grooves of a fracture-opening section.
The term “fracture groove” used in relation to the present invention refers to a weakened portion or a fracture portion for fracture opening which is composed of successive holes formed on the reverse or inside surface of an interior panel of an automobile through irradiation with a laser beam generated from laser generation means in the form of laser pluses.
2. Description of the Related Art
An airbag apparatus for a front passenger seat or a driver's seat of an automobile, such as a car, or that to be mounted on a side pillar of the automobile basically includes an airbag, an airbag case for accommodating the folded airbag, and an inflater for inflating the airbag. The airbag apparatus is disposed behind an interior panel of the automobile.
In general, an interior member of an automobile is composed of a panel core formed integrally from a plastic resin material, such as polypropylene, and an instrument panel (hereinafter referred to as an “interior panel”), which is disposed on the surface of the panel core and is composed of a single layer sheet or a multi-layer sheet. The single layer sheet is formed of a plastic resin. The multi-layer sheet is composed of a base layer formed from a plastic resin material, such as polypropylene, an intermediate foam layer, and a surface layer.
Conventionally, a seamless-type airbag apparatus for a front passenger seat has been provided (for example, see Japanese Patent Application Laid-Open (kokai) No. 2001-206180). As shown in FIGS. 1 and 2, the seamless-type airbag apparatus is designed to render fracture grooves 10a, 10b, and 10c of a fracture-opening section 14 invisible.
The airbag apparatus includes an airbag 12, an airbag case 13 for accommodating the airbag 12 in a folded condition, and an inflater (not shown) for inflating the airbag 12. The airbag apparatus is disposed inside an interior panel 10.
Specifically, as shown in FIGS. 1 and 2, the interior panel 10 has a fracture-opening section 14 having a size corresponding to that of an upper opening portion 13a of the airbag case 13. The fracture-opening section 14 is formed through provision of front and rear hinge grooves 10a extending longitudinally (in right-left direction), a center fracture groove 10b extending longitudinally, and side fracture grooves 10c extending transversely (in front-rear direction). These grooves 10a, 10b, and 10c are formed on the inside surface of the interior panel 10 by use of a laser. Specifically, a pulsating laser beam output from an unillustrated laser generation means is radiated perpendicular onto the inside surface of the interior panel 10 while being moved along peripheral edges of a pair of reinforcement plate members 11 fixed to the inside surface, thereby forming the grooves 10a, 10b, and 10c. 
The longitudinally extending center fracture groove 10b divides the fracture-opening section 14 into a front fracture-opening subsection 15a and a rear fracture-opening subsection 15b. Upon inflation of the airbag, the fracture-opening subsections 15a and 15b are split apart from each other in a casement condition (in opposite directions) while hinging on the front and rear hinge grooves 10a. 
When, upon inflation of the airbag, the fracture-opening section 14 of the interior panel 10 is split open by pressure of the inflating airbag 12, the front and rear hinge grooves 10a may be broken, with a resultant separation of the fracture-opening subsections 15a and 15b from the interior panel 10. In order to avoid this problem, the conventional air bag apparatus employs a reinforcing structure including the above-mentioned paired reinforcement plate members 11, which are provided on the inside surface of the fracture-opening section 14 of the interior panel 10. Specifically, a frame-shaped reinforcement rib 16 is formed integrally with the interior panel 10 in such a manner that the rib 16 projects from the inside surface of the interior panel 10, and the rib 16 defines a rectangular space slightly larger than the opening portion 13a of the airbag case 13. The paired reinforcement plate members 11 are disposed within the rectangular space defined by the rib 16. The horizontal portions 11a of the reinforcement plate members 11 are fixedly attached to the inside surface of the fracture-opening section 14 through thermal joining or like processing. Vertical portions 11b of the reinforcement plate members 11 are bent at their hinge portions 11c and extend downward along the inner surfaces of front and rear walls of the reinforcement rib 16. Openings 11d and 16a are formed in the front and rear walls of the reinforcement rib 16 and the corresponding vertical portions 11b of the reinforcement plate members 11, respectively. Hooks 17 attached to front and rear walls 13b of the airbag case 13 are engaged with the corresponding openings 11d and 16a. This structure prevents scattering of the fracture-opening subsections 15a and 15b of the fracture-opening section 14 of the interior panel 10 upon inflation of the airbag.
The thus-configured airbag apparatus functions in the following manner. Upon collision of the automobile, an impact force caused by the collision is detected by a sensor. A control unit including a CPU judges whether or not the detected impact force is equal to or greater than a predetermined value. When the control unit judges that the impact force is equal to or greater than the predetermined value, the control unit issues a signal for causing the inflater to generate a predetermined gas. The gas is fed to the airbag so as to promptly inflate the airbag.
The inflating airbag presses, from inside, the fracture-opening section 14 of the interior panel 10. The fracture-opening section 14 is fractured along the center fracture groove 10b and the side fracture grooves 10c, whereby the fracture-opening subsections 15a and 15b are split open about the front and rear hinge grooves 10a. The thus-opened front and rear fracture-opening subsections 15a and 15b are further opened while being turned inside out about the hinge portions 11c of the reinforcement plate members 11.
Simultaneously, the inflating airbag 12 projects outward from the interior panel 10 through the thus-opened fracture-opening section 14. Serving as a cushion, the inflated airbag 12 supports a front seat passenger at his/her chest and head, thereby protecting the passenger from the impact force of collision.
As described above, in the conventional seamless-type airbag apparatus, hinge grooves (weakened portions) and fracture grooves (for fracture opening) of the fracture-opening section are formed by use of a pulsating laser beam generated from laser generation means. Specifically, the front and back hinge grooves 10a, the center fracture groove 10b, and the side fracture grooves 10c are formed on the inside surface of the interior panel 10 through an operation of radiating a pulsating laser beam onto the inside surface of the interior panel 10 while moving the laser beam along peripheral edges of the pair of reinforcement plate members 11.
Therefore, in the case where an instrument panel is formed of a multi-layer sheet composed of a base layer, an intermediate foam layer, and a surface layer, tip ends of holes formed by means of laser machining may penetrate the surface layer, or reach the foam layer, whereby portions of the foam material in the vicinity of the holes melt. In such a case, depressions or machining marks are formed on the surface of the surface layer, with a resultant deterioration of the esthetic appearance.
Moreover, the center fracture groove 10b of the fracture-opening section 14 is weak against pressing force from the upper side, and a central portion or peripheral edge portion of the fracture-opening section 14 causes deformation such as subsidence. In such a case, shearing stresses act on the interior panel 10 when the airbag is expanded and deployed, and burrs are formed at the periphery of an opening formed upon fracture open of the fracture-opening section 14. In addition, the esthetic appearance of the interior panel is deteriorated considerably at the periphery of the fracture-opening section 14.